Activation needed every time change of phone company

Hi!

I have iphone4 some years ago, i "buy" it with a phone company, in Spain, near a year ago, i change my phone company before my first company make free my iphone, but when i put the new sim card i was need connect the phone to itunes and reactivate it.

In 2 weeks i will travel to asia, and want buy a prepaid card for this month, and need know if will need re-connect the phone to iTunes or just happends when you make the "liberation".

Sorry for my horrible spelling